1. Field of the Invention
This invention relates to semiconductor cascade light emitters (CLEs) in general and, more particularly, to broadband intersubband (ISB) [e.g., quantum cascade (QC)] lasers adapted for continuous wave (cw) operation.
2. Discussion of the Related Art
A broadband emitter has two principal characteristics: first, it simultaneously emits optical radiation (light) at multiple wavelengths, and second, adjacent emission wavelengths are sufficiently close to one another that their intensity does not fall to zero in the spectral region between them. The latter characteristic also means that the tails of the intensity profiles of adjacent wavelengths overlap one another. Broadband CLE devices include ISB cascade emitters, which usually operate in the mid-IR portion of the spectrum, and interband cascade emitters, which usually operate in the shorter IR range of about 1–5 μm.
ISB lasers are expected to be scientifically and technologically important for applications, such as chemical and biological sensing. In addition, due to its broad spectrum a broadband ISB laser has the potential to generate ultrashort pulses (e.g., below 1 ps) by mode- locking. Self-mode-locking of QC lasers based on the optical Kerr effect has been observed.
Dual wavelength QC lasers, for example, are described in U.S. Pat. No. 5,978,397 issued on Nov. 2, 1999 (Capasso et al. 43-74-7-11-8-12). This patent discloses, inter alia, a QC laser that utilizes a single type of radiative transition (RT) region (i.e., all RT regions are designed to emit at the same wavelength) but is field-tunable by means of a segmented electrode configuration. In one embodiment this QC laser was tunable over a range of about 6.2–6.6 μm, but required relatively complex circuitry.
Another illustration of a dual wavelength QC laser that utilizes a single type of RT region is described in U.S. Pat. No. 6,144,681 issued on Nov. 7, 2000 (Capasso et al. 48-80-12-15-12-17-1). This emitter is a 3-level device that emits light at two wavelengths by either of two mechanisms: (1) by a pair of vertical electron transitions at different wavelengths in a single quantum well, or (2) by a diagonal electron transition at one wavelength from one well into an adjacent well followed by a vertical electron transition at a different wavelength from the latter well. This source, also described by C. Sirtori et al. in Optics Lett., Vol. 23, No. 6, pp. 463–465 (1998), exhibited well-resolved luminescence peaks at wavelengths of 8 μm and 10 μm. However, the transitions were inefficient, and it was difficult to optimize both at the same time. Consequently, laser action was achieved on only one transition from the upper level to the middle level.
Yet another example of multi-wavelength ISB light emitters that utilize a single type of RT region is described in U.S. Pat. No. 6,148,012 issued on Nov. 14, 2000 (Capasso et al. 53-85-6-18-22-4). Here, the energy separation of the center wavelengths is greater than the largest line broadening energy associated with the emission wavelengths and means are provided for inhibiting the relaxation of electrons (e.g., the emission of optical phonons) from the upper to the lower energy levels associated with the radiative transitions. Illustrative emission spectra exhibited simultaneous lasing lines at about 6.3 μm, 7.3 μm and 7.9 μm.
In contrast, two wavelength operation has also been achieved in a QC laser having a heterogeneous cascade; i.e., a cascade that includes at least two different types of RT regions, each designed to emit radiation at a different wavelength. These devices are described in copending U.S. patent application Ser. No. 09/883,542 (Capasso et al 68-107-2-21-3-37) and by C. Gmachl et al. in Appl. Phys. Lett., Vol. 79, No. 5, pp. 572–574 (July 2001). In one embodiment, the heterogeneous cascade included two substacks that were optimized to emit at isolated wavelengths of 5.2 μm and 8.0 μm. Each substack was apportioned the optimum fraction of the applied bias voltage. This laser was not a broadband source; the intensity profiles of the 5.2 μm and 8.0 μm lines fell to zero in the spectral region between them.
Finally, a broadband QC laser emitting at wavelengths from 6 μm to 8 μm in a pulsed mode has recently been demonstrated by C. Gmachl et al., as reported in Nature, Vol. 415, pp. 883–887 (February 2002). A number of dissimilar ISB optical transitions were made to cooperate in order to provide broadband optical gain from 5 μm to 8 μm. This laser had 36 stages with radiative transition (RT) regions each centered at a different emission wavelength. The stages of RT regions and I/R (injection/relaxation) regions were designed to compensate for the wavelength dependent losses and achieve flat net gain over the desired wavelength region of operation. However, discrepancies between calculations and experiments were significant and caused considerable variation of the net modal gain across the spectrum that prevented broadband cw operation of these lasers.
The publications discussed above, as well as the patents and applications (all of which are assigned to the assignee hereof), are incorporated herein by reference.
Thus, a need remains in the art for a broadband ISB laser that is capable of cw operation.
A need also remains for broadband CLEs in which the difference in intensity between different wavelengths is reduced (i.e., the intensity or gain profile is flattened).